In a wireless communication system, a receiving end and a transmission end usually employ space division multiplexing in a multi-antenna system to obtain a higher transmission rate. In order to realize space division multiplexing, the transmission end usually maps a data layer to be transmitted to an antenna port by utilizing precoding, thus obtaining high transmission performance. In order to realize the precoding on the transmission end, the transmission end needs to know all or a part of the channel state information. In a Time Division Duplexing (TDD) system, a transmission end may obtain the channel state information via the reciprocity of uplink and downlink channels; and in a Frequency Division Duplex (FDD) system, the receiving end usually needs to notify the transmission end of the channel state information.
In a Long Term Evolution (LTE) system, the channel state information fed back by a terminal usually contains a Channel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI) and a Rank Indication (RI). The CQI represents channel quality information, and a base station configures a Modulation and Coding Scheme (MCS) adapted to the channel quality according to the CQI; the RI represents the Rank of a channel, and the base station configures a number of suitable data transmission layers based on the RI. The PMI represents a precoding matrix recommended by the terminal, and the base station selects a suitable precoding matrix according to the PMI. As specified in the protocol, after a terminal selects the precoding matrix, the terminal feds back the PMI in an agreed format. The base station may restore, via this format, the precoding matrix selected by the terminal, and this agreed format is referred to as a codebook. In early LTE versions, Discrete Fourier Transform (DFT) vectors can be used as a codebook. That is, a terminal generates a certain number of DFT vectors according to parameters of high-layer signaling configuration, then selects among them a better DFT vector therefrom according to the RI and the channel estimation result, and reports an index number thereof as the PMI to the base station; the base station then can restore the DFT vector recommend by the terminal according to the PMI. With the evolution of LTE, the codebook is further enhanced: the PMIs fed back are divided into long-term feedback and short-term feedback, wherein the long-term feedback keeps constant in bandwidth, and the PMI value of long-term feedback may determine a DFT vector combination, the short-term feedback selects a suitable DFT vector from the DFT vector set, and the PMI value changes with the subband; after a dual polarized antenna is involved, the same DFT vector is selected in the two polarization directions, and only polarization phase difference is involved. In Rel-13 (Version 13), a concept of Frequency Division (FD) Multiple-Input Multiple Output (MIMO) is involved, so that the antenna array of the base station is allowed to be arranged in a 2D topology, and the maximum number of supportable ports is increased to 16, the codebook is enhanced to a form of DFT vector Kronecker product. The PMI includes three parts: a horizontal DFT vector of long-term feedback, a vertical DFT vector of long-term feedback, and a subband feedback for column and phase difference selection; the base station generates a group of DFT vectors as result of the vertical long-term feedback and generates another group of DFT vectors as result of the horizontal long-term feedback. Then a new vector group is generated by obtaining Kronecker product of every two DFT vectors in the two groups of DFT vectors. Finally, a precoding matrix can be constructed, by selecting suitable columns from the newly generated vector group according to the subband feedback result and adding the polarization phase difference. In such the PMI construction and feedback mode, the channel information in the vertical and horizontal dimensions may be utilized simultaneously, and the feedback overhead may be reduced to a certain degree.
However, after the amount of antenna ports is extended to 32, 64 or more, the above method will have several limitations:
a) There are many forms on possible antenna topologies, so that it will increase complexity if a certain codebook is designed for each of the antenna topologies;
b) There are many vectors in a precoding vector set generated via long-term feedback, so that if suitable vectors are to be selected therefrom for feedback, a large feedback overhead will be required; and
c) After the scale of antenna arrays is increased, the correlativity between channels cannot be well quantized via the DFT vector-based precoding construction mode.
Based on the above limitations, when the codebook construction and feedback mode in Rel-13 are applied to 32 or more antenna ports, a large feedback overhead will be caused; an and also the corresponding performance gain cannot be attained.